The present invention relates generally to systems and methods that employ X-ray sources.
X-ray sources have found widespread application in devices such as imaging systems. X-ray imaging systems utilize an X-ray source in the form of an X-ray tube to emit an X-ray beam which is directed toward an object to be imaged. The X-ray beam and the interposed object interact to produce a response that is received by one or more detectors. The imaging system then processes the detected response signals to generate an image of the object.
For example, in typical computed tomography (CT) imaging systems, an X-ray tube projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system and generally referred to as the “imaging plane”. The X-ray beam passes through the object being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated radiation beam received at the detector array is dependent upon the attenuation of the X-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile.
In known third-generation CT systems, the X-ray tube and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged so that the angle at which the X-ray beam intersects the object constantly changes. A group of X-ray attenuation measurements, i.e. projection data, from the detector array at one gantry angle is referred to as a “view”. A “scan” of the object comprises a set of views made at different gantry angles during one revolution of the X-ray source and detector. In an axial scan, the projection data is processed to construct an image that corresponds to a two-dimensional slice taken through the object.
Conventional X-ray tubes comprise a vacuum vessel, a cathode assembly, and an anode assembly. The vacuum vessel is typically fabricated from glass or metal, such as stainless steel, copper or a copper alloy. The cathode assembly and the anode assembly are enclosed within the vacuum vessel.
To generate an X-ray beam, the cathode emits electrons which are then accelerated toward the anode, causing the electrons to impact a target zone of the anode at high velocity. The acceleration is caused by a voltage difference (typically, in the range of 20 kV to 140 kV for medical purposes, although possibly higher or lower especially for non-medical purposes) which is maintained between the cathode and anode assemblies. The X-rays emanate from a focal spot of the target zone in all directions, and a collimator is then used to direct X-rays out of the vacuum vessel in the form of an X-ray fan beam toward the patient.
In typical X-ray tubes, electrons are emitted from the cathode by a process known as thermionic emission. According to this process, the cathode filament (which is typically formed of a tungsten wire) is provided a current that causes resistive heating of the filament to high temperatures. At such temperatures, the electrons in the filament have sufficient energy that they do not bond to specific atoms (the energy level of the electrons places the electrons in the conduction band) and therefore are susceptible to being emitted from the cathode. A complex focusing structure is used to direct the electrons toward the focal spot.
A problem that is therefore encountered is that the cathode is continuously provided with electrical energy which is converted to heat energy, and it is necessary to remove the heat energy from the cathode. Removing heat energy from the cathode is difficult, however, because the cathode is located inside the vacuum vessel and therefore convection is not available as a heat transfer mechanism. Additionally, although conduction is available as a heat transfer mechanism, the large voltage differential that is maintained between the cathode and the anode results in the construction of the cathode being undesirably complex, especially when taken in combination with the complex focusing mechanism that is also provided. A more significant problem is that the heat causes the filament to move (thermal expansion) and changes the location and shape of the focal spot on the target.
Therefore, an improved X-ray source which reduces the need for heat transfer away from the cathode and which is relatively simple in construction would be highly advantageous.